High surface area, electrically conductive nanocarbon-supported metal oxide

ABSTRACT

A metal oxide-carbon composite includes a carbon aerogel with an oxide overcoat. The metal oxide-carbon composite is made by providing a carbon aerogel, immersing the carbon aerogel in a metal oxide sol under a vacuum, raising the carbon aerogel with the metal oxide sol to atmospheric pressure, curing the carbon aerogel with the metal oxide sol at room temperature, and drying the carbon aerogel with the metal oxide sol to produce the metal oxide-carbon composite. The step of providing a carbon aerogel can provide an activated carbon aerogel or provide a carbon aerogel with carbon nanotubes that make the carbon aerogel mechanically robust.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 61/147,805 filed Jan. 28, 2009entitled “High Surface Area, Electrically ConductiveNanocarbon-Supported Metal Oxide,” the disclosure of which is herebyincorporated by reference in its entirety for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

BACKGROUND

1. Field of Endeavor

The present invention relates to metal oxide and more particularly to ahigh surface area, electrically conductive nanocarbon-supported metaloxide.

2. State of Technology

Porous metal oxides can be prepared by a number of techniques rangingfrom sol-gel synthesis to various templating/support methods. Theseporous metal oxides have shown enhanced catalytic activity, compared tobulk material, but are still limited by surface areas less than 1000m²/g. This is even the case when using high surface area templates suchas SBA-15 or MCM-41. Surface areas for the templated metal oxides can beless than 200 m²/g. The use of supports, such as carbon nanotubes, alsoyields surface areas less than 300 m²/g. Another issue presented by manyporous metal oxides is that their pore structure collapsing at elevatedtemperatures. For example in titania aerogels, this lack of porestability results in order of magnitude decreases in surface area underheating. The presence of silica has been shown to provide somestabilization of pores at high temperatures in titania-silicacomposites. However, the surface area is still significantly decreasedunder heating.

Carbon nanotubes (CNTs) possess a number of intrinsic properties thathave made them promising materials in the design of composite materials.CNTs can have electrical conductivities as high as 10⁶ Sm⁻¹, thermalconductivities as high as 3000 Wm⁻¹K⁻¹, elastic moduli³ on the order of1 TPa, and are extremely flexible. Unfortunately, the realization ofthese properties in macroscopic forms such as foams and composites hasbeen limited. Foams, though conductive, tend to be mechanically weak dueto their dependence on van der Waals forces for mechanical integrity.

The treatise, Introduction to Nanotechnology, by Charles P. Poole, Jr.,and Frank J. Owens. John Wiley &. Sons, 2003, states: “Nanotechnology isbased on the recognition that particles less than the size of 100nanometers (a nanometer is a billionth of a meter) impart tonanostructures built from them new properties and behavior. This happensbecause particles which are smaller than the characteristic lengthsassociated with particular phenomena often display new chemistry andphysics, leading to new behavior which depends on the size. So, forexample, the electronic structure, conductivity, reactivity, meltingtemperature, and mechanical properties have all been observed to changewhen particles become smaller than a critical size.”

SUMMARY

Features and advantages of the present invention will become apparentfrom the following description. Applicants are providing thisdescription, which includes drawings and examples of specificembodiments, to give a broad representation of the invention. Variouschanges and modifications within the spirit and scope of the inventionwill become apparent to those skilled in the art from this descriptionand by practice of the invention. The scope of the invention is notintended to be limited to the particular forms disclosed and theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

The present invention provides a metal oxide-carbon aerogel compositethat includes a carbon aerogel with a metal oxide overcoat. The metaloxide-carbon composite is made by providing a carbon aerogel, immersingthe carbon aerogel in a metal oxide sol under a vacuum, returning thecarbon aerogel with the metal oxide sol to atmospheric pressure, curingthe carbon aerogel with the metal oxide sol at room temperature, anddrying the carbon aerogel with the metal oxide sol to produce the metaloxide-carbon composite. The step of providing a carbon aerogel can beproviding an activated carbon aerogel or providing a carbon aerogel withcarbon nanotubes that make the carbon aerogel mechanically robust.

The invention has use as a commercial catalyst. The invention also hasuse as an electrode, for example as an electrode for batteries and supercapacitors. The invention also has use in water purification,electrical/electrochemical energy storage, solar energy, and hydrogenstorage.

The invention is susceptible to modifications and alternative forms.Specific embodiments are shown by way of example. It is to be understoodthat the invention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of the specification, illustrate specific embodiments of theinvention and, together with the general description of the inventiongiven above, and the detailed description of the specific embodiments,serve to explain the principles of the invention.

FIGS. 1A and 1B are SEM and TEM images of TiO₂/SWNT-CA.

FIG. 2 is a TGA plot of SWNT-CA, TiO₂/SWNT-CA, and TiO₂ in air.

FIG. 3 is Semi-log plot of the pore size distribution of the SWNT-CA,TiO₂/SWNT-CA, and TiO₂ aerogel.

FIGS. 4A-D are SEM images of TiO₂/CNT (a,b) and TiCN/CNT (c,d) atdifferent magnifications.

FIGS. 5A and 5B are TEM images of TiO₂/CNT and (b) TiCNT/CNT.

FIGS. 6A-H are SEM images of ACA (a,b), as-prepared TiO₂/ACA (c,d),heat-treated TiO₂/ACA (e,f), and TiCN/ACA (g,h) at differentmagnifications. Arrows indicate particles of amorphous (d), crystallineTiO₂ (f), and TiCN (h).

FIGS. 7A-C are transmission electron microscopy images of as-preparedTiO₂/ACA (a), heat-treated TiO₂/ACA (b), and TiCN/ACA (c).

FIGS. 5A-D are SEM images of as-prepared SiO₂/ACA and SiC/ACA.

FIG. 9 is a flow chart showing one embodiment of a method of making ametal oxide-carbon composite with carbon nanotubes that make said metaloxide-carbon composite mechanically robust.

FIG. 10 is a flow chart showing one embodiment of a method of making anmetal oxide-carbon composite with an activated carbon aerogel.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring to the drawings, to the following detailed description, and toincorporated materials, detailed information about the invention isprovided including the description of specific embodiments. The detaileddescription serves to explain the principles of the invention. Theinvention is susceptible to modifications and alternative forms. Theinvention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

The present invention provides a metal oxide-carbon composite thatincludes a carbon aerogel with an oxide overcoat. The metal oxide-carboncomposite is made by providing a carbon aerogel, immersing the carbonaerogel in a metal oxide sol under a vacuum, returning the carbonaerogel with the metal oxide sol to atmospheric pressure, curing thecarbon aerogel with the metal oxide sol-gel at room temperature toproduce the metal oxide-carbon wet gel composite, and drying the metaloxide-carbon wet gel composite to produce the metal oxide-carbon aerogelcomposite. The step of providing a carbon aerogel can be providing anactivated carbon aerogel or providing a carbon aerogel with carbonnanotubes that make the carbon aerogel mechanically robust. Apparatusand method of providing an aerogel and a metal oxide are described inU.S. Pat. No. 6,986,818, U.S. Pat. No. 7,270,851; U.S. Pat. No.7,410,718; U.S. Published Patent Application No. 20090123358; PublishedPatent Application No. 20090229032; and Published Patent Application No.20090317619. U.S. Pat. No. 6,986,818, U.S. Pat. No. 7,270,851; U.S. Pat.No. 7,410,718; U.S. Published Patent Application No. 20090123358;Published Patent Application No. 20090229032; and Published PatentApplication No. 20090317619 are incorporated herein in their entirety bythis reference for all purposes.

DEFINITION OF TERMS

Various terms used in this patent application are defined below.

CA=Carbon Aerogel

CNT=Carbon Nanotubes

CA-CNT=Carbon Aerogel & Carbon Nanotube Composite

SWNT=Single-Walled Carbon Nanotubes

DWNT=Double-Walled Carbon Nanotubes

SDBS=Sodium Dodecylbenzene Sulfonate

MESOPORPOUS=Pore Dia. 2-50 nm

PVA=Polyvinyl Alcohol

CVD=Chemical Vapor Deposition

TEM=Transmission Electron Microscopy

SEM=Scanning Electron Microscopy

R/C=Resorcinol to Catalyst Ratios

RF=Resorcinol and Formaldehyde Solids

BET=Brunauer-Emmett-Teller

Mechanically Robust=Can withstand strains greater than 10% beforefracture

Electrically Conductive=Exhibits an electrical conductivity of 10 S/m orgreater

Ultralow-Density=Exhibits densities less than 50 mg/cc

Carbon Nanotube-Based Aerogel=Porous carbon material consisting of 5 to95% carbon nanotubes by weight

EXAMPLES

The present invention provides a method of making a metal oxide-carboncomposite, comprising the steps of providing an aqueous media or othermedia to form a suspension, adding reactants and catalyst to saidsuspension to create a reaction mixture, curing said reaction mixture toform a wet gel, drying said wet gel to produce a dry gel, pyrolyzingsaid dry gel to produce an aerogel, immerse said aerogel in a metaloxide sol under a vacuum, returning said aerogel and said metal oxidesol to atmospheric pressure, curing said aerogel at room temperature,and drying said aerogel producing an aerogel oxide composite. The metaloxide-carbon composite comprises a carbon aerogel, said carbon aerogelhaving inner surfaces, and an oxide coating said inner surfaces of saidcarbon aerogel providing an aerogel oxide composite. In one embodimentthe carbon aerogel is a carbon aerogel with carbon nanotubes that makesaid carbon aerogel mechanically robust. In another embodiment thecarbon aerogel is an activated carbon aerogel. In one embodiment theoxide is titanium oxide. In another embodiment the oxide is an oxidefrom transitional metal oxide made with forming precursors of manganeseor iron or cobalt or nickel or copper or zinc or zirconium or tin saltsor alkoxides.

Example Nanocarbon-Supported Titanium Dioxide

The present invention provides the fabrication of newnanocarbon-supported titanium dioxide structures that exhibit highsurface area and improved electrical conductivity. Nanocarbonsconsisting of single-walled carbon nanotubes and carbon aerogelnanoparticles were used to support titanium dioxide particles andproduce monoliths with densities as low as 80 mg/cm³. The electricalconductivity of the nanocarbon-supported titanium dioxide was dictatedby the conductivity of the nanocarbon support while the pore structurewas dominated by the titanium dioxide aerogel particles. Theconductivity of the monoliths presented here was 72 S/m and the surfacearea was 203 m²/g.

Titanium dioxide is a widely researched material with applicationsranging from photocatalysts to electrodes to hydrogen storage materials.However, issues such as absorption limited to the ultraviolet range,high rates of electron-hole recombination, and relatively low surfaceareas have limited commercial use of titanium dioxide. Recent effortshave focused on combining titanium dioxide with various materials toaddress some of these issues. Titanium dioxide in the presence of carbon(e.g. carbon nanotubes (CNT)) is currently one of the most attractivecombinations. While recent work has shown some improvements, surfacesareas and photocatalytic activity are still limited. Maintaining highsurface areas while improving electrical conductivities, one couldenvision charging-discharging rates and photoefficiencies that aresignificantly higher than currently possible. Unfortunately for CNTcomposites, improvements in electrical conductivity are often not fullyrealized due to poor dispersion of CNTs in the matrix material, impedingthe formation of a conductive network. However, with a mechanicallyrobust, electrically conductive CNT foam, one could imagine simplycoating this low-density CNT scaffold with titanium dioxide, yieldingconductive nanocarbon-supported titanium dioxide.

Here Applicants present the synthesis and characterization of such ahigh-surface area, conductive TiO₂/CNT composite. Applicants recentlyreported the synthesis of a novel CNT-based foam, consisting of bundlesof single-walled nanotubes (SWNT) crosslinked by carbon aerogel (CA)nanoparticles, which would serve as an excellent candidate for the CNTscaffold of the TiO₂/CNT composite. The SWNT-CA foams simultaneouslyexhibited increased stiffness, and high electrical conductivity even atdensities approaching 10 mg cm⁻³ without reinforcement. The foams arestable to temperatures approaching 1000° C. and have been shown to beunaltered by exposure to extremely low temperatures during immersion incryogenic liquids. So, in addition to their use in applications such ascatalyst supports, sensors, and electrodes, these ultralight, robustfoams could allow the formation of novel CNT composites. As theconductive network is already established, it can be impregnated throughthe wicking process with a matrix of choice, ranging from inorganic solsto polymer melts to ceramic pastes. Thus, a variety of conductive CNTcomposites could be created using the SWNT-CA foam as a pre-made CNTscaffold. Applicants use the SWNT-CA as a scaffold for the synthesis ofconductive, high surface area TiO₂/CNT composites.

Experiment

Materials. All reagents were used without further purification.Resorcinol (99%) and formaldehyde (37% in water) were purchased fromAldrich Chemical Co. Sodium carbonate (anhydrous) was purchased from J.T. Baker Chemical Co. Highly purified SWNTs were purchased from CarbonSolutions, Inc.

SWNT-CA preparation. The SWNT-CAs were prepared as described in previouswork. Briefly, in a typical reaction, purified SWNTs (Carbon Solutions,Inc.) were suspended in deionized water and thoroughly dispersed using aVWR Scientific Model 75T Aquasonic (sonic power −90 W, frequency −40kHz). The concentration of SWNTs in the reaction mixture was 0.7 wt %.Once the SWNTs were dispersed, resorcinol (1.235 g, 11.2 mmol),formaldehyde (1.791 g, 22.1 mmol) and sodium carbonate catalyst (5.95mg, 0.056 mmol) were added to the reaction solution. The resorcinol tocatalyst ratios (R/C) employed was 200. The amount of resorcinol andformaldehyde (RF solids) used was 4 wt %. The sol-gel mixture was thentransferred to glass molds, sealed and cured in an oven at 85° C. for 72h. The resulting gels were then removed from the molds and washed withacetone for 72 h to remove all the water from the pores of the gelnetwork. The wet gels were subsequently dried with supercritical CO2 andpyrolyzed at 1050° C. under a N2 atmosphere for 3 h. The SWNT-CAsmaterials were isolated as black cylindrical monoliths. Foams with SWNTloadings of 30 wt % (0.5 vol %) were prepared by this method.

TiO₂/SWNT-CA composite preparation. Sol-gel chemistry was used todeposit the TiO₂ aerogel layer on the inner surfaces of the SWNT-CAsupport. The TiO₂ sol-gel solution was prepared as described in previouswork, In a typical synthesis, SWNT-CA parts were immersed in the TiO₂sol-gel solution and full infiltration of the SWNT-CA pore network bythe sol-gel solution was achieved under vacuum. Following gelation ofthe titania network, the wet composite was dried using supercriticalCO₂, yielding the TiO₂/SWNT-CA composite.

Characterization. Bulk densities of the TiO₂/SWNT-CA composites weredetermined from the physical dimensions and mass of each sample. Thevolume percent of SWNT in each sample was calculated from the initialmass of SWNTs added, assuming a CNT density of 1.3 g/cm³, and the finalvolume of the aerogel. Scanning electron microscopy (SEM)characterization was performed on a JEOL 7401-F at 10 keV (20 mA) in SEImode with a working distance of 2 mm. Transmission electron microscopy(TEM) characterization was performed on a JEOL JEM-200CX.Thermogravimetric analysis (TGA) was performed on a Shimadzu TGA 50Thermogravimetric Analyzer to determine TiO₂ content. Samples wereheated in flowing air at 10 seem to 1000° C. at 10° C. min in aluminaboats. The weight fraction of material remaining was assumed to be purestoichiometric TiO₂. Energy dispersive spectroscopy confirmed that onlyTiO₂ remained after TGA was performed. Surface area determination andpore volume and size analysis were performed by Brunauer-Emmett-Teller(BET) and Barrett-Joyner-Halenda (BJH) methods using an ASAP 2000Surface Area Analyzer (Micromeritics Instrument Corporation). Samples ofapproximately 0.1 g were heated to 300° C. under vacuum (10 Ton) for atleast 24 hours to remove all adsorbed species. Electrical conductivitywas measured using the four-probe method similar to previous studies.Metal electrodes were attached to the ends of the cylindrical samples.The amount of current transmitted through the sample during measurementwas 100 mA and the voltage drop along the sample was measured overdistances of 3 to 6 mm. Seven or more measurements were taken on eachsample.

The microstructure of the TiO₂/SWNT-CA composites was examined using SEMand TEM. As shown in FIG. 1A and FIG. 1B, the network structure of theTiO₂/SWNT-CA composites is similar to that observed in pristine SWNT-CA.The presence of the TiO₂ aerogel layer on the surface of the nanotubebundles can be seen in TEM image. Interestingly, the TiO₂ aerogelappears to have formed primarily on the surfaces of the nanotube bundlesdespite the fact that the TiO₂ sol-gel solution filled the entire porevolume of the support. The open pore volume in the TiO₂/SWNT-CAcomposite is only sparsely populated with TiO₂ particles. Thisobservation indicates that nucleation of the TiO₂ particles during thesol-gel reaction preferentially occurs at the surface of the nanotubebundles.

Thermal gravimetric analysis in air was used to determine the TiO₂content in the as-TiO₂/SWNT-CA composites as illustrated in FIG. 2. Asexpected, combustion of the pristine SWNT-CA occurs around 500° C. andthe material is completely consumed by 600° C. The 5 wt % remaining islikely metal catalyst from the CNTs. The titania exhibits an initialmass loss generally attributed to moisture and organics below 300° C.and is stable thereafter. Not surprisingly, the TGA plot forTiO₂/SWNT-CA material is a composite of the plots for titania and theSWNT-CA. It is interesting to note that the combustion of the SWNT-CAoccurs significantly earlier for the TiO₂/SWNT-CA compared to that forthe pristine SWNT-CA, which may be the result of a catalytic effect ofthe titania aerogel particles on carbon oxidation. Nevertheless, thenearly 50 wt % remaining after combustion of the SWNT-CA confirm thepresence of titania in the TiO₂/SWNT-CA composite.

FIG. 3 plots the pore size distribution of the SWNT-CA, TiO₂/SWNT-CAcomposite, and pristine TiO₂ aerogel. The BET surface area, electricalconductivity and other physical properties of these materials aresummarized in Table I. Table I shows that the TiO₂/SWNT-CA composite hashigh surface area and electrical conductivity. In fact, the electricalconductivity of the SWNT-CA is not adversely affected by theinfiltration of the insulating material. Though, based on the SEM andTEM images (FIG. 1), the titania aerogel appears to simply coat theSWNT-CA scaffold, the increased surface area suggests that the poremorphology of the titania dominates the overall pore morphology of thecomposite. This is confirmed via the pore size distribution, which showsthat the pore size distribution of the TiO₂/SWNT-CA is much closer tothat of pristine TiO₂ aerogel than that of the SWNT-CA. Thus, with theTiO₂/SWNT-CA composite, a new class of materials with good electricalconductivity and high surface area are realized.

TABLE 1 Physical properties of SWNT-CA, TiO₂/SWNT-CA, and TiO₂ aerogel.CNT, vol % Density, S_(BET), σ, Material (wt %) g/cm³ m²/g Scm⁻¹ SWNT-CA0.5 (30) 0.030 184 0.77 TiO₂/SWNT-CA 0.5 (8)  0.082 203 0.72 TiO₂aerogel  0 (0) 0.193 237 <0.001

Applicants have described a straightforward method for the fabricationof electrically conductive, high-surface area TiO₂/CNT composites. Thenovel TiO₂/SWNT-CA monoliths was prepared by coating the CNT strutswithin the SWNT-CA scaffold with amorphous sol-gel-derived TiO₂particles. Given the technological interest in crystalline TiO₂, work isin progress to convert the amorphous TiO₂ layer to the anatasecrystalline phase. The conductive network of the SWNT-CA scaffoldremained intact after infiltration yielding a composite with aconductivity of 72 S m-l and a surface area of 203 in 2 g”]. Therefore,the SWNT-CAs were shown to provide the means to create conductive,high-surface area TiO₂ composites. The general nature of this methodshould provide a route for the synthesis of a variety of conductive,high surface area composites with applications in photocatalysts andenergy storage.

This nanocarbon-supported titanium dioxide example is described ingreater detail in the journal article, “Synthesis and Characterizationof Nanocarbon-Supported Titanium Dioxide,” Author(s): Marcus A Worsley,Joshua D. Kuntz, Octavio Cervantes, T Yong-Jin Han, Peter Pauzauskie,Joe H Satcher, Theodore F Baumann, Paper #: 1174-V03-06, DOI:10.1557/PROC-1174-V03-06, 2010 MRS Spring Meeting, Material ResearchSociety. The journal article “Synthesis and Characterization ofNanocarbon-Supported Titanium Dioxide,” by Marcus A. Worsley, Joshua D.Kuntz, Octavio Cervantes, T. Yong-Jin Han, Peter J. Pauzauskie, Joe H.Satcher, Jr. and Theodore F. Baumann, Mater. Res. Soc. Proc. Vol. 1174,(2009) is incorporated herein in its entirety by this reference for allpurposes.

Example High Surface Area Carbon Nanotube-Supported TitaniumCarbonitride Aerogels

Porous transition metal nitrides and carbides have received considerableattention recently as catalysts and catalyst supports. They exhibit highresistance to sintering and poisoning, in addition to catalytic activityfor a number of useful reactions. Of particular interest is the factthat these transition metal compounds have been shown to have catalyticactivity similar to that of typical noble metal catalysts. Thus,substituting transition metal compounds for noble metals is anattractive option for reducing the cost of catalyst materials.Unfortunately, traditional routes to forming metal nitrides andcarbides, such as the carbothermal reduction of metal oxides, yield lowsurface area materials. To increase the specific surface area oftransition metal carbides and nitrides, a number of new syntheticmethods have been proposed. One promising approach involves the use ofhigh surface area templates or supports to control the microstructure ofthe transition metal nitride and carbide. For example, both high surfacearea SiO₂ and C₃N₄ have been used to form TiN powders with surface areasin excess of 100 m²/g. With surface areas as high as 1000 m²/g, carbonnanotubes (CNT) could also serve as such a high surface area support.There have been a number of studies exploring the deposition of variousmetal oxides on CNTs, however, to our knowledge, only one study examinesdepositing a transition metal nitride on CNTs. And while the fabricationof metal nitride or carbide nanostructures has received a lot ofattention, the use of CNTs for creating high surface area transitionmetal nitrides or carbides has not been reported.

Here Applicants report the synthesis and characterization of amonolithic CNT-supported titanium carbonitride aerogel (TiCN/CNT) withsurface area in excess of 250 m²/g. This TiCN/CNT was formed by thecarbothermal reduction of a TiO₂-coated low-density CNT-based foam(TiO₂/CNT) in flowing nitrogen. The CNT-based foam (30 wt % CNT, 30 mgcm⁻³) that serves as the support consists of single-walled carbonnanotubes crosslinked by carbon aerogel particles (SWNT-CA), aspreviously described. To prepare the TiO₂/CNT, the SWNT-CA was immersedin a TiO₂ sol under vacuum prior to gelation, similar to the methodpreviously reported for fabricating stiff, conductive polymer/CNTcomposites. The TiO₂ sol was prepared via a two-step sol-gel processinvolving the acid-catalyzed hydrolysis of titanium tetraethoxide,followed by base-initiated gelation of the TiO₂ species. Briefly, asolution of titanium tetraethoxide (1.0 g, 4.4 mmol) and pure ethanol(4.5 mL) was prepared in an ice bath with vigorous stirring. Oncechilled, hydrochloric acid (37%, 71.4 μL) and deionized water (85.7 μL)were then added to the titanium tetraethoxide/ethanol solution. Afterfive minutes of continuous stirring, propylene oxide (0.36 g, 6.1 mmol)was finally added to the reaction mixture. The reaction mixture wasstirred for another five minutes before immersing the SWNT-CA monolithin the TiO₂ sol. Vacuum was applied to the reaction vessel to ensurecomplete infiltration of the TiO₂ sol in the SWNT-CA. Afterinfiltration, the TiO₂ sol was then allowed to gel in the SWNT-CA underambient conditions. The wet composite gel was then dried usingsupercritical CO₂, yielding the TiO₂/CNT. The TiO₂/CNT was then heatedunder flowing nitrogen at 1400° C. for 4 hours to yield the TiCN/CNTmonolith.

Powder X-ray diffraction (XRD) analysis of the samples was performedwith Cu Kα radiation on a Scintag PAD-V X-ray diffractometer. TiO₂powder was used as a standard. Bulk densities of the monoliths weredetermined from the physical dimensions and mass of each sample.Scanning electron microscopy (SEM) and energy-dispersive X-rayspectroscopy (EDX) characterization were performed on a JEOL 7401-F at5-10 keV (20 mA) in SEI mode with a working distance of 2-8 mm. Tosupplement EDX, thermogravimetric analysis (TGA) was performed on aShimadzu TGA 50 Thermogravimetric Analyzer. Samples were heated in airto 1000° C. at 10° C./min in alumina boats. Transmission electronmicroscopy (TEM) characterization was performed on a JEOL JEM-200CXElectron Microscope operated at 200 kV. Samples for TEM were prepared bypulverizing aerogels above TEM grids. Surface area determination andpore volume and size analysis were performed by Brunauer-Emmett-Teller(BET) and Barrett-Joyner-Halenda (BJH) methods using an ASAP 2000Surface Area Analyzer (Micromeritics Instrument Corporation). Samples ofapproximately 0.1 g were heated to 300° C. under vacuum (10⁻⁵ Torr) forat least 24 hours to remove all adsorbed species prior to analysis.Electrical conductivity was measured using the four-probe method similarto previous studies. Metal electrodes were attached to the ends ofcylindrical samples. The amount of current transmitted through thesample during measurement was 100 mA, and the voltage drop along thesample was measured over distances of 3 to 6 mm.

SEM images of the TiO₂/CNT, FIG. 4A and FIG. 4B and TiCN/CNT) FIG. 4Cand FIG. 4D show the ligament and pore structure of these materials. TheTiO₂/CNT resembles the CNT-based foam except for the coating ofamorphous TiO₂. The TEM image of the TiO₂/CNT, FIG. 2A, supports thisview. The TiCN/CNT also has the same basic structure as the originalCNT-based foam except that the ligaments are now decorated with TiCNnanocrystals FIGS. 4B and 4C. This observation suggests that the carbonconsumed during the reduction of TiO₂ comes primarily from the carbonaerogel coating the CNT bundles, leaving the CNTs intact. The integrityof the CNTs was also confirmed via Raman spectroscopy throughobservation of the peaks characteristic of CNTs (ESI†) in the TiCN/CNT.The TEM image, FIG. 2B, also shows that the TiCN/CNT ligaments, onaverage, have smaller diameters than the TiO₂/CNT. The smaller diametersprobably occur as the TiO₂ is reduced and carbon aerogel is consumed inthe course of forming the TiCN nanocrystals. The TiCN/CNT had a brownishcolor compared to the jet-black CNT-based foam and TiO₂/CNT.

TABLE 1I Density (ρ), electrical conductivity (σ), and elemental content(Ti, C, N, O) of the composite foams Material ρ, g cm⁻³ σ, S cm⁻¹ Ti, at% (wt %) C, at % (wt %) N, at % (wt %) O, at % (wt %) CNT-based foam0.030 0.77 — 95 (93) — 5.0 (6.6) TiO₂/CNT 0.082 0.72 9.4 (28) 71 (53) —19 (19) TiCN/CNT 0.055 0.25 17 (43) 65 (43) 18 (14) <1 (<1)

Table II summarizes some basic properties of the TiCN/CNT, as well asthe CNT-based foam and the TiO₂/CNT. The density of the TiCN/CNT issignificantly reduced compared to the TiO₂/CNT. During the carbothermalreduction, the monolith experienced 49% mass loss and 28% volumeshrinkage, resulting in the 55 mg cm⁻³ final density. The electricalconductivity of the TiCN/CNT is diminished compared to the CNT-basedfoam and TiO₂/CNT, but still high considering the extremely low bulkdensity of the TiCN/CNT foam. The partial consumption during the heattreatment of the graphitic carbon aerogel particles that crosslink theCNT bundles, is likely the cause of the decreased conductivity.Interfacial resistance has been shown to be a dominant factor in thetransport properties of CNT composites. The removal or narrowing of thecritical conduction pathways between CNT bundles effectively increasesthe interfacial resistance, leading to a decrease in the bulkconductivity.

Elemental analysis by EDX and TGA suggests that the TiO₂ in the TiO₂/CNTis completely converted to TiCN in the TiCN/CNT. This observation isconsistent with literature on the carbothermal reduction of TiO₂ underthe conditions of this study. Under a constant supply of nitrogen andexcess carbon, it is expected that 100% reduction should occur, assumingtemperature and time are chosen appropriately. Previous studies haveshown 100% reduction at temperatures as low as 1300° C. for a 4 hourhold time. The roughly 1:1 Ti:N ratio suggests a fairly N-rich TiCNphase was formed. EDX elemental mapping (ESI†) shows an evendistribution of elements indicative of a TiCN layer that covers most ofthe CNT surface. XRD analysis offers more details concerning thecomposition of the TiCN phase.

Powder XRD was used to determine what phases were present in theTiCN/CNT. For reference, XRD patterns of the CNT-based foam and TiO₂/CNTwere also included. The largest peaks from the CNT-based foam can beattributed to the (100) and (101) graphite peaks (PDF #41-1487). Thesepeaks are also visible in the pattern from the TiO₂/CNT. The absence ofadditional peaks in the TiO₂/CNT pattern supports the earlier suggestionthat the TiO₂ coating the CNT ligaments is amorphous. The XRD peaks forthe TiCN/CNT would indicate the presence of the osbornite crystallinephase of TiCN (PDF #06-642). The calculated lattice parameter, a, forthe TiCN/CNT, 4.244 Å, is in good agreement with TiC_(1-x)N_(x) (x=0.95)and very close to the value for pure TiN, 4.240. Peak broadeningindicates that the average crystallite size is about 20 nm, consistentwith the particle sizes observed in SEM and TEM analysis and. Therefore,based on the XRD data, a highly nitrogen-enriched layer of TiCNnanocrystals covers the CNT bundles

Nitrogen adsorption/desorption analysis was performed to determinesurface area, pore volume and average pore size of the TiCN/CNT. Allthree samples had Type IV nitrogen isotherms (ESI†), indicative of thepredominantly macroporous nature of the CNT-based foam that serves asthe foundation for all the samples. The addition of TiO₂ and theconversion to TiCN increased both the surface area and pore volume ofthe composite foams. Peak pore size increases from 56 nm in theCNT-based foam to 72 nm in the TiO₂/CNT and TiCN/CNT. The TiO₂/CNTexhibits pore morphology similar to that of an amorphous TiO₂ aerogel,suggesting that the TiO₂ coating the CNT bundles dominates the nitrogensorption behavior. The TiCN/CNT maintains the same general morphology asthe TiO₂/CNT, as evidenced by a similar pore size distribution. However,the surface area and pore volume are increased because of the decreasedbulk density and additional porosity due to removal of carbon (in theform of gaseous CO) that occurs during carbothermal reduction. Similarincreases in surface area were observed by Berger et al. under similarconditions during the conversion of TiO₂ (rutile) and carbon (furnaceblack or graphite).

In summary, the synthesis of high surface area TiCN/CNT has been shownby the carbothermal reduction of TiO₂ in a CNT-based foam. The resultingmonolith was conductive, contained N-rich TiCN nanocrystals decoratingCNT bundles and had a surface area of 276 m²/g. The straightforwardnature of this method should allow for the synthesis of other highsurface area CNT-supported metal nitrides (e.g. ZrN, Si₃N₄) by simplyreducing the respective oxide (e.g. ZrO₂, SiO₂). Also, by performing thecarbothermal reduction in inert gas (e.g. Ar), high surface areacarbides (e.g. TiC, SiC) could also be formed. Thus, a new class ofmonolithic, high surface area CNT-supported carbides and nitrides couldbe developed with potential for significant contributions inapplications such as catalysis.

This high surface area carbon nanotube-supported titanium carbonitrideaerogels example is described in greater detail in the journal article“High surface area carbon nanotube-supported titanium carbonitrideaerogels,” by Marcus A. Worsley, Joshua D. Kuntz, Peter J. Pauzauskie,Octavio Cervantes, Joseph M. Zaug, Alex E. Gash, Joe H. Satcher Jr., andTheodore F. Baumann, Journal of Materials Chemestry, 2009, 19,5503-5506. The in the journal article “High surface area carbonnanotube-supported titanium carbonitride aerogels,” by Marcus A.Worsley, Joshua D. Kuntz, Peter J. Pauzauskie, Octavio Cervantes, JosephM. Zaug, Alex E. Gash, Joe H. Satcher Jr., and Theodore F. Baumann,Journal of Materials Chemestry, 2009, 19, 5503-5506 is incorporatedherein in its entirety by this reference for all purposes.

Example High Surface Area TiO₂/C and TiCN/C Composites

Nanocomposites of titania and various forms of carbon (i.e. carbonnanotubes, activated carbons, ordered carbons, etc.) exhibit a number ofenhanced functional properties for catalysis and energy-storageapplications. Several reports have shown that titania/carbon (TiO₂/C)composites have higher photocatalytic activity, improvedphotoefficiency, and a wider absorption band than titania alone.Composites of TiO₂/C have also been shown to improve the energy andpower density of electrochemical cells and enhance the storage capacityand reversibility of hydrogen-storage materials. The efficacy of thesecomposite materials depends mainly on the crystallinity and surface areaof the titania species. As a result, significant efforts have beenfocused on the design of high surface area composites containing eitherrutile or anatase TiO₂. One approach to the fabrication of thesecomposites has been the incorporation of the titania within high surfacearea supports or scaffolds. While this approach has generated a varietyof novel titania composites, the surface areas of the composites aretypically lower than those of the scaffolds themselves. The decrease insurface area is generally attributed to blocking of the micropores inthe support by the deposited titania, decreasing the accessible surfacearea. The design of a high surface area support containing bimodalporosity (macro- and micropores) could limit the detrimental effectsassociated with pore-plugging, thereby providing a route to a new classof high surface area titania composites.

Applicants recently reported the synthesis of activated carbon aerogel(ACA) monoliths that exhibited hierarchical porosity and surface areasin excess of 3000 m² g⁻¹. In this article, Applicants use thesematerials as scaffolds for the synthesis of high surface area titaniaand titanium carbonitride (TiCN) composites. The composites are preparedthrough coating the inner surfaces of monolithic ACA templates with alayer of sol-gel-derived titania, yielding the TiO₂/ACA composite. In atypical synthesis, ACA parts were immersed in the TiO₂ sol-gel solutionand full infiltration of the ACA pore network by the sol-gel solutionwas achieved under vacuum. After drying, the amorphous TiO₂ overcoat inthe composite can then be converted to either anatase TiO₂ or titaniumcarbonitride through heat treatment under different conditions. Toconvert the amorphous TiO₂ layer to anatase, the as-prepared. TiO₂/ACApart was heated in air at 400° C. for 2 hours. Alternatively, to preparethe TiCN-coated ACA composite, the as-prepared TiO₂/ACA part was heatedunder flowing nitrogen at 1400° C. for 4 hours. In both cases, theheat-treated composite materials exhibit extremely high BET surfaceareas (>1800 m² g⁻¹) and retain the porous network structure of themonolithic ACA support. Because of the technological importance oftitania and its well-documented conversion to TiC_(1-x)N_(x) (0<x<1) viacarbothermal reduction, these systems were chosen to demonstrate thepotential of the ACA as a scaffolding material. Nevertheless, theapproach described here is general and can be applied to the fabricationof other high surface area metal oxide, metal nitride and metal carbidecomposites of interest.

The microstructures of the titania-ACA composites were evaluated usingscanning electron microscopy FIGS. 6A-D and transmission electronmicroscopy FIGS. 7A-C. SEM images of as-prepared TiO₂/ACA FIGS. 6C-Dshow the same trabecular structure and texture as observed in thepristine ACA FIGS. 6A-B. The presence of the TiO₂ aerogel layer on thesurface of the ACA can be seen in images of the as-prepared TiO₂/ACAcomposites. Interestingly, the TiO₂ aerogel appears to have formedprimarily on the surfaces of the ACA despite the fact that the TiO₂sol-gel solution filled the entire pore volume of the support. As seenin FIGS. 7C-D and FIG. 7A, the open pore volume in the ACA composite isonly sparsely populated with TiO₂ particles. This observation indicatesthat nucleation of the TiO₂ particles during the sol-gel reactionpreferentially occurs at the surface of the ACA. After heat treatment at400° C., the texture of the TiO₂/ACA composite appears to roughen,apparently due to the formation of anatase TiO₂ nanocrystals on the ACAsurface FIG. 6E-F and FIG. 7C. Further changes in texture are seen aftercarbothermal reduction of the surface layer of TiO₂ to TiCN FIG. 6G-Hand FIG. 7C. In the TiCN/ACA composite, cubic TiCN crystals ranging insize from 10 to 100 nm are clearly visible on the ACA surface. Thecontinuous nature of the crystalline TiCN layer suggests that thedeposited TiO₂ completely coated the entire surface of the ACA support.With the bulk of the TiO₂ deposited at the ACA surface, the number ofTiO₂ particles formed in sol filling the free space in the ACA isgreatly reduced.

Thermal gravimetric analysis in air was used to determine the TiO₂content in the as-prepared and annealed TiO₂/ACA composites as well asthe TiCN content in the TiCN/ACA composite. As expected, combustion ofthe pristine ACA begins oxidizing at 400° C. and the material iscompletely consumed by 600° C. The onset of mass loss for the annealedTiO₂/ACA composite is similar to that of the ACA, but the materialretains 20% of its original mass due to the presence of the TiO₂overcoat Table III. In contrast to the ACA and TiO₂/ACA materials, theTiCN/ACA composite exhibits a slight weight gain at ˜350° C. prior tocombustion of the carbon support. The increase in mass can be attributedto oxidation of the TiCN layer (molecular weight of 60-62) to TiO₂(molecular weight of 80). Interestingly, complete oxidation of the ACAsupport in the TiCN/ACA composite does not occur until 680° C. ascompared to 600° C. for the other samples, suggesting that the TiCNcompletely covers the ACA surface, providing an effective barrier tooxygen diffusion. In addition, the energy dispersive X-ray spectroscopy(EDX) element mapping of the TiCN/ACA shows an even distribution of Ti,C, and N, consistent with a TiCN layer covering most of the ACA, asobserved in the SEM and TEM images. Only after the TiCN is converted tothe oxide does combustion of the ACA occur. The remaining 18 wt % TiO₂from combustion of the TiCN/ACA composite implies a starting TiCNcontent of 14 wt %.

TABLE III Physical properties for the ACA support, TiO₂ aerogels and theACA composites Monolithic TiO₂/ density/g S_(RET)/ V_(total)/ V_(micro)/Mateiral wt % cm

3 m³ g

cm³ g

cm³ g

ACA  0 0.140 2455 1.05 0.42 TiO₂ aerogel 78 0.193 237 0.53 — (asprepared) TiO₂ aerogel 99 n.a.^(a) 141 0.33 — (heat-treated) TiO₂/ACA 150.230 1507 0.91 0.50 (as-prepared) TiO₂/ACA 20 0.104 2054 1.30 0.61(heat-treated) TiCN/ACA 14^(b) 0.148 1838 1.01 0.43 ^(a)The heat-treatedTiO₂ aerogel was isolated as a powder, ^(b)TiCN content shown forTiCN/ACA.

indicates data missing or illegible when filed

Powder XRD was used to determine the crystalline phases of theheat-treated TiO₂/ACA and TiCN/ACA composites. For comparison, the XRDpattern of the ACA was also included. The XRD pattern for theas-prepared TiO₂/ACA (no heat treatment) was very similar to that of theACA, likely due to the amorphous nature of the titania, and is,therefore, not shown. The largest peaks in the diffraction pattern forthe ACA material can be attributed to the (100) and (101) graphite peaks(PDF #41-1487). These peaks are also visible in the diffraction patternsfor the heat-treated TiO₂/ACA and TiCN/ACA composites due to thepresence of the ACA support. The remaining peaks in the XRD pattern forthe annealed TiO₂/ACA composite can be indexed to the anatase phase ofTiO₂ (PDF #21-1272). Analysis of the peaks using the Scherrer equationindicates the average crystallite size is ˜9 nm, in agreement with thesmall size of the crystals observed by electron microscopy. The XRDpeaks for the TiCN/ACA composite indicate the presence of the osbornitecrystalline phase of TiCN (PDF #06-0642) on the ACA support. Thecalculated lattice parameter, a, for the TiCN in the TiCN/ACA, 4.248 Å,is in good agreement with TiC_(1-x)N_(x) (x=0.90) and very close to thevalue for pure TiN, 4.240. The high nitrogen content is consistent withEDX results showing a Ti:N ratio of close to one. The averagecrystallites size calculated from the XRD data (˜20 nm) correlates withthe size range of the cubic crystals observed in SEM and TEM analysis.Therefore, based on the XRD data, the heat-treated TiO₂/ACA compositecontained purely anatase nanocrystals, and full reduction of TiO₂ toTiCN was achieved in the TiCN/ACA composite to create a highlynitrogen-enriched layer of TiCN nanocrystals on the ACA surface.

The textural properties of the TiO₂/ACA and TiCN/ACA composites wereevaluated using nitrogen adsorption/desorption analysis Table III. Forcomparison, data for the ACA and TiO₂ aerogel (before and after heattreatment) are also included in Table III. Nitrogenadsorption/desorption plots for the ACA and the composites. Each of thecomposites exhibited type II nitrogen isotherms, indicating a mostlymacroporous (<2 nm) material with the remaining pore volume primarily inthe large meso- and macropore (>90 nm) range. Coating of the ACAframework with TiO₂ clearly results in a significant decrease in BETsurface area (1507 m² g⁻¹) relative to the uncoated ACA. Nevertheless,the surface area of the as-prepared TiO₂/ACA composite represents almostan order of magnitude improvement over that of the as-prepared TiO₂aerogel. Retention of such a large BET surface area in the coatedmaterial suggests that the ACA is less susceptible to the negativeeffects of pore-plugging observed in other scaffold materials, such asactivated carbons. Additionally, heat treatment of the as-preparedTiO₂/ACA leads to a 36% increase in surface area in the annealedcomposite (2054 m² g⁻¹). This observation is in contrast to the sharpdecrease in surface area that occurs upon annealing of the bulk TiO₂aerogel prepared without the scaffold. The increased surface area andpore volume in the annealed composite indicate that the ACA supportprevents coarsening and collapse of the TiO₂ coating during heattreatment, even as the amorphous titania is converted to the anatasephase. The presence of high-surface area SiO₂ has been shown to havesimilar effects on the temperature stability of pores in TiO₂ gels. Inaddition, correspondingly lower density of the annealed TiO₂/ACA(relative to as-prepared TiO₂/ACA) is consistent with a lack of porecollapse and likely contributes to the observed textural properties.Similarly, the TiCN/ACA composite also exhibits increased surface areaand pore volume relative to the as-prepared TiO₂/ACA composite TableIII. The increased surface area can be attributed to the additionalporosity created by the removal of carbon from the ACA support (in theform of gaseous CO) that occurs during carbothermal reduction. Similarincreases in surface area have been reported under similar conditionsduring the conversion of TiO₂ (rutile) and carbon (furnace black orgraphite) mixtures to TiCN. While the surface area and pore volume forthe TiCN/ACA composite are slightly lower than those of the heat-treatedTiO₂/ACA, the textural properties are still quite close to those of theoriginal ACA. This observation demonstrates the flexibility of the ACAscaffold for creating a variety of high surface area oxide, carbide andnitride materials.

In a typical synthesis, titanium(IV) ethoxide (1 g, 0.0125 mol) andethanol (3.57 g, 0.0776 mol), hydrochloric acid (71.4 μl), and water(851 μl) were mixed in an ice bath, followed by the addition ofpropylene oxide (0.357 g, 0.00616 mol) to prepare the titania sol. Anactivated carbon aerogel monolith was immersed in the titania sol in aglass vial and held under vacuum to ensure full penetration of the solin the carbon aerogel. The reaction mixture was then cured at roomtemperature for 24 h. The wet composite was washed in ethanol and driedby supercritical extraction in CO₂ to yield the TiO₂/ACA composite.Annealing the as-prepared TiO₂/ACA composite in air at 400° C. for 2 hwas required to convert the amorphous titania layer on the ACA to theanatase phase. Alternatively, heating the as-prepared TiO₂/ACA compositein flowing nitrogen at 1400° C. for 4 h produced the TiCN/ACA composite.

Powder X-ray diffraction (XRD) analysis of the samples was performedwith Cu Kα radiation on a Scintag PAD-V X-ray diffractometer. TiO₂(anatase) powder was used as a standard. Bulk densities of the monolithswere determined from the physical dimensions and mass of each sample.Scanning electron microscopy (SEM) and energy-dispersive X-rayspectroscopy (EDX) characterization was performed on a JEOL 7401-F at5-10 keV (20 mA) in SEI mode with a working distance of 2-8 mm.Transmission electron microscopy (TEM) characterization was performed ona JEOL JEM-200CX electron microscope operated at 200 kV.Thermogravimetric analysis (TGA) was performed on a Shimadzu TGA 50thermogravimetric analyzer to determine TiO₂ and TiCN contents. Sampleswere heated in flowing air at 10 sccm to 1000° C. at 10° C. min⁻¹ inalumina boats. The weight fraction of material remaining was assumed tobe pure stoichiometric TiO₂. The TiCN content of the TiCN/ACA wascalculated from the weight fraction of TiO₂ remaining after heating to1000° C. in air assuming full oxidation of initial TiCN content. Energydispersive spectroscopy confirmed that only TiO₂ remained after TGA wasperformed. Surface area determination and pore volume analysis wereperformed by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda(BJH) methods using an ASAP 2000 surface area analyzer (MicromeriticsInstrument Corporation). Samples of approximately 0.1 g were heated to300° C. under vacuum (10⁻⁵ Torr) for at least 24 h to remove alladsorbed species, prior to analysis.

In conclusion, the synthesis and characterization of TiO₂/ACA andTiCN/ACA composites with the highest surfaces areas yet reported hasbeen described. The flexibility of the described method should allow forsynthesis of other high surface area metal oxides, carbides, andnitrides through the use of supports with bimodal porosity, like theACA, to minimize pore-plugging effects. This new class of high-surfacearea materials should be especially advantageous in technologies such ascatalysis and energy storage where high surface area and accessible porevolume are desired.

This high surface area TiO₂/C and TiCN/C composites example is describedin greater detail in the journal article “high surface area TiO₂/C andTiCN/C composites,” by Marcus A. Worsley, Joshua D. Kuntz, OctavioCervantes, T. Yong-Jin Han, Alex E. Gash, Joe H. Satcher, Jr andTheodore F. Baumann, Journal of Materials Chemestry, 2009, 19,7146-7150. The journal article “high surface area TiO₂/C and TiCN/Ccomposites,” by Marcus A. Worsley, Joshua D. Kuntz, Octavio Cervantes,T. Yong-Jin Han, Alex E. Gash, Joe H. Satcher, Jr and Theodore F.Baumann, Journal of Materials Chemestry, 2009, 19, 7146-7150 isincorporated herein in its entirety by this reference for all purposes.

Example Improved Thermal Stability for High Surface Area SiO₂/C andSiC/C Composites

The synthesis and characterization of high surface area carbon-supportedsilica and silicon carbide aerogels are described. An activated carbonaerogel with surface area greater than 3000 m²/g was used to as asupport for the sol-gel deposition of silica. The resultingsilica-coated carbon aerogel retained a surface area greater than 2000m²/g and showed improved thermal stability in air. The carbon-supportedsilicon carbide aerogel was made by the carbothermal reduction of thesilica-coated carbon aerogel under flowing Ar at 1500° C. The resultingmonolith maintained a surface area greater than 2000 m²/g and was stableto temperatures approaching 600° C., over 100° C. higher than that ofthe pristine carbon aerogel.

The microstructures of the silica-ACA composites were evaluated usingscanning electron microscopy FIGS. 8A and 8B. SEM images of as-preparedSiO₂/ACA. FIGS. 8A and 8B show the same trabecular structure and textureas observed in the pristine ACA. The presence of the SiO₂ aerogel layeron the surface of the ACA can be seen in images of the as-preparedSiO₂/ACA composites. Interestingly, the SiO₂ aerogel appears to haveformed primarily on the surfaces of the ACA despite the fact that theSiO₂ sol-gel solution filled the entire pore volume of the support. Asseen in FIGS. 8A and 8B, the open pore volume in the ACA composite isonly sparsely populated with SiO₂ particles. This observation indicatesthat nucleation of the SiO₂ particles during the sol-gel reactionpreferentially occurs at the surface of the ACA. Further changes intexture are seen after carbothermal reduction of the surface layer ofSiO₂ to SiC (FIGS. 8C and 8D). In the SiC/ACA composite, virtually noparticles are visible in the open pore volume. In fact, the SiC/ACAappears to have the same texture as the pristine ACA suggesting the SICforms a fairly conformal layer on the ACA. Similar results were observedin the case of TiO₂ and TiCN on ACA.

Energy dispersive x-ray analysis was used to track the compositionchange of the composite during the carbothermal reduction. Oxygen atomiccontent was used to determine the level of reduction as the SiO₂/ACA waspopulated with SiO₂ particles. This observation indicates thatnucleation of the SiO₂ particles during the sol-gel reactionpreferentially occurs at the surface of the ACA. Further changes intexture are seen after carbothermal reduction of the surface layer ofSiO₂ to SiC (FIG. 8A-D). In the SiC/ACA composite, virtually noparticles are visible in the open pore volume. In fact, the SiC/ACAappears to have the same texture as the pristine ACA suggesting the SiCforms a fairly conformal layer on the ACA. Similar results were observedin the case of TiO₂ and TiCN on ACA.

The carbothermal reduction was considered complete when the O content inthe solid phase is reduced to zero. At 1500° C. the O content drops from12 at % to 3 at % within the first 10 minutes suggesting formation of anSi_(x)O_(y)C_(x) phase. The Si and C content show correspondingincreases during this initial period. The O content then slowlydecreases to zero over the next 5 h. The Si and C content remain fairlyconstant. Based on these results, it was concluded that a 5 h treatmentat 1500° C. was sufficient to completely convert the SiO₂ layer in theSiO₂/ACA to SIC. This is consistent with literature on SiC synthesis.

Powder XRD was used to confirm the presence of SiC in the SiC/ACAcomposite. For comparison, the XRD pattern of the as-prepared SiO₂/ACAwas also included. The XRD pattern for the pristine ACA is identical tothat of the SiO₂/ACA, due to the amorphous nature of the as-preparedsilica, and is, therefore, not shown. The largest peaks in thediffraction pattern for the SiO₂/ACA material can be attributed to the(100) and (101) graphite peaks. These peaks are also visible in thediffraction pattern for the SiC/ACA composites due to the presence ofthe ACA support. The remaining peaks in the XRD pattern for the SiC/ACAcomposite can be indexed to moissanite SiC. Analysis of the peaks usingthe Scherrer equation indicates the average crystallite size is ˜26 nm.Therefore, based on the XRD and EDX data, full reduction of SiO₂ to SiCwas achieved in the SiC/ACA composite to create a layer of SiCnanocrystals on the ACA surface.

Thermal gravimetric analysis in air was used to determine the thermalstability of the SiO₂/ACA and SiC/ACA, as well as the SiO₂ and SiCcontent. As expected, combustion of the pristine ACA begins at 400° C.and the material is completely consumed by 600° C. The mass loss eventbelow 200° C. for the SiO₂/ACA is due to organic impurities from theas-prepared SiO₂. The onset of ACA mass loss for the SiO₂/ACA compositeis ˜100° C. higher than that of the pristine ACA, suggesting that theSiO₂ covers the ACA surface fairly well and forms a decent barrier tooxygen diffusion. Similar improvements in thermal stability were notedwith a TiCN/ACA. In the case of TiCN/ACA, the TiCN was completelyoxidized to TiO₂ in the process, in contrast to the SiO₂ in theSiO₂/ACA. For the SiO₂/ACA, complete oxidation of the ACA occurs at 690°C. This material retains 15% of its original mass due to the presence ofthe SiO₂ overcoat. Further improvements in thermal stability areobserved in the SiC/ACA composite. Mass loss does not begin until closeto 600° C. and complete oxidation of the carbon support does not occuruntil 720° C. Like the SiO₂/ACA, this improved thermal stabilitysuggests that the SiC completely covers the ACA surface, providing aneffective barrier to oxygen diffusion. The remaining 10% materialremaining represents oxidation-resistant SiC.

The textural properties of the SiO₂/ACA and SiC/ACA composites wereevaluated using nitrogen adsorption/desorption analysis (Table 1). Eachof the composites exhibited type II nitrogen isotherms, indicating amostly microporous (<2 nm) material with the remaining pore volumeprimarily in the large meso- and macropore (>90 nm) range. Coating ofthe ACA framework with SiO₂ clearly results in a significant decrease inBET surface area (2288 m²/g) relative to the uncoated ACA. Nevertheless,the surface area of the as-prepared SiO₂/ACA composite represents almostan order of magnitude improvement over that of the as-prepared SiO₂aerogel. Retention of such a large BET surface area in the coatedmaterial suggests that the ACA is less susceptible to the negativeeffects of pore-plugging observed in other scaffold materials, such asactivated carbons.

After carbothermal reduction, the textural properties show littlechange. There is small loss of surface area and pore volume, likely dueto sintering that occurs during the reduction process. While the surfacearea and pore volume for the SiC/ACA composite are slightly lower thanthose of the heat-treated SiO₂/ACA, the textural properties are stillquite close to those of the original ACA. This observation demonstratesthe effectiveness of the ACA scaffold for creating high surface areaoxide and carbide materials.

Experimental

In a typical synthesis, trimethoxysilane (IV) ethoxide (4.1 g) andmethanol (14 g), ammonium hydroxide (200 ml), and water (1.5 g) weremixed to prepare the silica sol. An activated carbon aerogel monolithwas immersed in the silica sol in a glass vial and held under vacuum toensure full penetration of the sol in the carbon aerogel. The reactionmixture was then cured at room temperature for 24 h. The wet compositewas washed in ethanol and dried by supercritical extraction in CO₂ toyield the SiO₂/ACA composite. Heating the as-prepared SiO₂/ACA compositein flowing argon at 1500° C. for 5 h produced the SiC/ACA composite.

Powder x-ray diffraction (XRD) analysis of the samples was performedwith Cu K_(a) radiation on a Scintag PAD-V X-ray diffractometer. TiO₂(anatase) powder was used as a standard. Bulk densities of the monolithswere determined from the physical dimensions and mass of each sample.Scanning electron microscopy (SEM) and energy-dispersive x-rayspectroscopy (EDX) characterization was performed on a JEOL 7401-F at5-10 keV (20 mA) in SEI mode with a working distance of 2-8 mm.Transmission electron microscopy (TEM) characterization was performed ona JEOL JEM-200CX Electron Microscope operated at 200 kV.Thermogravimetric analysis (TGA) was performed on a Shimadzu TGA 50Thermogravimetric Analyzer to determine SiO₂ and SiC content. Sampleswere heated in flowing air at 10 sccm to 1000° C. at 10° C./min inalumina boats. The weight fraction of material remaining was assumed tobe pure stoichiometric SiO₂ and SiC. Surface area determination and porevolume analysis were performed by Brunauer-Emmett-Teller (BET) andBarrett-Joyner-Halenda (BJH) methods using an ASAP 2000 Surface AreaAnalyzer (Micromeritics Instrument Corporation).⁴² Samples ofapproximately 0.1 g were heated to 300° C. under vacuum (10⁻⁵ Torr) forat least 24 h to remove all adsorbed species, prior to analysis.

The synthesis and characterization of SiO₂/ACA and SiC/ACA compositeswith the highest surfaces areas yet reported has been described. Theflexibility of the described method should allow for synthesis of otherhigh surface area metal oxides, carbides, and nitrides through the useof supports with bimodal porosity, like the ACA, to minimizepore-plugging effects. This new class of high-surface area materialsshould be especially advantageous in technologies such as catalysis andenergy storage where high surface area and accessible pore volume aredesired.

Referring now to FIG. 9 a flow chart illustrates one embodiment of amethod of making a carbon aerogel oxide composite in accordance with thepresent invention. The method is designated generally by the referencenumber 900. The method 900 includes a number of steps. The steps includedispersing nanotubes in an aqueous media or other media to form asuspension, adding reactants and catalyst to the suspension to create areaction mixture, curing the reaction mixture to form a wet gel, dryingthe wet gel to produce a dry gel, pyrolyzing the dry gel to produce acarbon nanotube-based aerogel, immerse the carbon nanotube-based aerogelin a metal oxide sol under a vacuum, returning the carbon nanotube-basedaerogel and the metal oxide sol to atmospheric pressure, curing themetal oxide-carbon nanotube-based composite at room temperature, anddrying the metal oxide-carbon nanotube-based wet gel composite producingan metal oxide-carbon composite. In one embodiment the step of immersingthe carbon nanotube-based aerogel in a metal oxide sol under a vacuumcomprises immersing the carbon nanotube-based aerogel in titaniumdioxide. In one embodiment the step of immersing the carbonnanotube-based aerogel in a metal oxide sol under a vacuum comprisesimmersing the carbon nanotube-based aerogel in a metal oxide sol madefrom Mn, Fe, Co, Ni, Cu, Sn, Al, Si, Zn, Zr sol-gel precursors incombination with catalyst, and sol-gel forming components. Referringagain to FIG. 9, the method 900 includes a number of steps. The stepsshown include the steps described below.

Step number 901 is “Obtain resorcinol, form-aldehyde, sodium carbonate,sodium dodecylbenzene sulfonate (SDBS) and purified double-wallednanotubes (DWNT).”

Step number 902 is “Purified DWNTS suspended in aqueous solutioncontaining SDBS.”

Step number 903 is “Dispersal of DWNTS in aqueous surfactant solutioncontaining SDBS using soniction.”

Step number 904 is “Resorcinol, formaldehyde and sodium carbonatecatalyst added to the reaction solution.”

Step number 905 is “Sol-Gel mixture transferred to glass molds sealedand cured in oven at 85° C. for 72 hours.”

Step number 906 is “Resulting gel removed from mold and washed withacetone for 72 hours to remove all water from pores of gel network.”

Step number 907 is “Wet gel dried with supercritical CO₂ and pyrolyzedat 1050° C. under N₂ atmosphere for 3 hours.”

Step number 908 is “Resulting composite material (CA-DWNT) isolated asblack cylinder monoliths.”

Step number 909 is “Immerse in titanium dioxide (Ti 0₂) sol:infiltration of pore network achieved under vacuum.”

Step number 910 is “Return to atmospheric pressure and dry wet compositeusing supercritical CO₂ producing a metal oxide-carbon composite.

Referring now to FIG. 10 a flow chart illustrates an embodiment of amethod of making a metal oxide-carbon aerogel composite in accordancewith the present invention. The method is designated generally by thereference number 1000. The method 1000 includes a number of steps. Thesteps include providing an aqueous media or other media to form asuspension, adding reactants and catalyst to the suspension to create areaction mixture, curing the reaction mixture to form a wet gel, dryingthe wet gel to produce a dry gel, pyrolyzing the dry gel to produce anaerogel, immerse the aerogel in a metal oxide sol under a vacuum,returning the aerogel and the metal oxide sol to atmospheric pressure,curing the metal oxide sol-infiltrated carbon aerogel, and drying themetal oxide-carbon wet gel composite producing a metal oxide-carbonaerogel composite. In one embodiment the step of immersing the carbonaerogel in a metal oxide sol under a vacuum comprises immersing thecarbon aerogel in titanium dioxide sol. In one embodiment the step ofimmersing the carbon aerogel in a metal oxide sol under a vacuumcomprises immersing the carbon aerogel in a metal oxide sol made fromMn, Fe, Co, Ni, Cu, Zn, Zr sol-gel precursors in combination with acatalyst, and sol-gel forming components.

Referring again to FIG. 10, the method 1000 includes a number of steps.The steps shown include the steps described below.

Step number 1001 is “Resorcinol and 37% formaldehyde solution dissolvedin water.”

Step number 1002 is “Add glacial acetic acid.”

Step number 1003 is “Transferred to glass molds and cured at 80° C. for72 hours.”

Step number 1004 is “Resultant organic hydrogels washed with acetone toremove water and dried with supercritical C0².”

Step number 1005 is “Organic aerogels carbonized at 1050° C. for 3 hoursunder N₂ atmosphere.”

Step number 1006 is “Carbon monoliths.”

Step number 1007 is “Activating carbon aerogel by exposing to stream ofCO₂ at 950° for different soak times.”

Step number 1008 is “Shorter activation time new porosity is in the formof micropores.”

Step number 1009 is “Longer activation time. The micropore are widenedto sizes that cross the micropore mesopore boundry.”

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

1. A method of making a metal oxide-carbon composite, comprising thesteps of: providing a carbon aerogel, immersing said carbon aerogel in ametal oxide sol under a vacuum, returning said carbon aerogel with saidmetal oxide sol to atmospheric pressure, curing said carbon aerogel withsaid metal oxide sol to produce a metal oxide-carbon wet gel composite,and drying said metal oxide-carbon wet gel composite so producing ametal oxide-carbon composite.
 2. The method of making a metaloxide-carbon composite of claim 1 wherein said step of providing acarbon aerogel comprises providing an activated carbon aerogel.
 3. Themethod of making a metal oxide-carbon composite of claim 1 wherein saidstep of providing a carbon aerogel comprises providing a carbon aerogelcarbon aerogel with carbon nanotubes that make said carbon aerogelmechanically robust.
 4. The method of making a metal oxide-carboncomposite of claim 1 wherein said step of immersing said carbon aerogelin a metal oxide sol under a vacuum comprises immersing said carbonaerogel in titanium dioxide.
 5. The method of making a metaloxide-carbon composite of claim 1 wherein said step of immersing saidcarbon aerogel in a metal oxide sol under a vacuum comprises immersingsaid carbon aerogel in a metal oxide sol made from Mn, Fe, Co, Ni, Cu,Zn, Zr salts in combination with propylene oxide, and sol-gel formingcomponents.
 6. The method of making a metal oxide-carbon composite ofclaim 1 wherein said step of immersing said carbon aerogel in a metaloxide sol under a vacuum comprises immersing said carbon aerogel in ametal oxide sol for metal species including but not limited tomanganese, iron, cobalt, nickel, copper, zinc, zirconium, tin, aluminumand chromium.
 7. A method of making a metal oxide-carbon composite,comprising the steps of: providing an aqueous media or other media toform a suspension, adding reactants and catalyst to said suspension tocreate a reaction mixture, curing said reaction mixture to form a wetgel, drying said wet gel to produce a dry gel, pyrolyzing said dry gelto produce an aerogel, immerse said aerogel in a metal oxide sol under avacuum, returning said aerogel and said metal oxide sol to atmosphericpressure, curing said sol, and drying said sol-gel producing a metaloxide-carbon composite.
 8. A metal oxide-carbon composite, comprising: acarbon aerogel, said carbon aerogel having inner surfaces, and an oxidecoating said inner surfaces of said carbon aerogel providing a metaloxide-carbon composite.
 9. The metal oxide-carbon composite of claim 8wherein said carbon aerogel is a carbon aerogel with carbon nanotubesthat make said carbon aerogel mechanically robust.
 10. The metaloxide-carbon composite of claim 8 wherein said carbon aerogel is anactivated carbon aerogel.
 11. The metal oxide-carbon composite of claim8 wherein said oxide is titanium oxide.
 12. The metal oxide-carboncomposite of claim 8 wherein said oxide is an oxide from metal oxidemade with forming precursors including but not limited to manganese oriron or cobalt or nickel or copper or zinc or zirconium or aluminum orsilicon or tin salts or alkoxides.
 13. A metal oxide-carbon composite,comprising: a carbon aerogel with carbon nanotubes that make said carbonaerogel mechanically robust, said carbon aerogel having inner surfaces,and an oxide coating said inner surfaces of said carbon aerogelproviding an metal oxide-carbon composite.
 14. A metal oxide-carboncomposite, comprising: an activated carbon aerogel, said activatedcarbon aerogel having inner surfaces, and an oxide coating said innersurfaces of said activated carbon aerogel providing an metaloxide-carbon composite.